Zoological Studies 43(2): 165-172 (2004) Potential Use of Extremely High Biomass and Production of Copepods in an Enclosed Brackish Water Body in Lake Nakaumi, Japan, for the Mass Seed Production of Fishes Shin-ichi Uye1,*, Shinobu Nakai1 and Moriyuki Aizaki2 1Graduate School of Biosphere Sciences, Hiroshima University, 4-4-1 Kagamiyama, Higashi-Hiroshima 739-8528, Japan Tel: 81-824-247940. Fax: 81-824-227059. E-mail: [email protected] 2Faculty of Life and Environmental Sciences, Shimane University, Matsue 690-8504, Japan Tel: 81-852-326582. Fax: 81-852-326598. E-mail: [email protected] (Accepted January 3, 2004) Shin-ichi Uye, Shinobu Nakai and Moriyuki Aizaki (2004) Potential use of extremely high biomass and pro- duction of copepods in an enclosed brackish water body in lake Nakaumi, Japan, for the mass seed production of fishes. Zoological Studies 43(2): 165-172. We found extremely high abundance, biomass, and production rates of mesozooplankton in an enclosed brackish water body (called the Honjo District) in Lake Nakaumi, Japan, during a 2-yr investigation. To the best of our knowledge, the overall biomass mean of 71.0 mg C m-3 is one of the highest values recorded so far anywhere in the world. Copepods dominated the zooplankton com- munity in terms of abundance (94.4%) and biomass (83.4%). Zooplankton biomass and production rates were twice as high in Honjo District as those in adjacent Lake Nakaumi, although the phytoplankton chlorophyll α concentration was twice as low. Two reasons for the enhanced zooplankton standing stock in Honjo District might be the development of weak benthic deoxygenation and lower numbers of planktivorous fish. We pro- pose to use zooplankton as a food source for the intensive mass seed production of finfish. By our conserva- tive estimate, exploiting 10% of the daily zooplankton production (or around 2.5% of the biomass) of Honjo District would allow the production of 5.6 million red sea bream (Pagrus major) or Japanese flounder (Paralichthys olivaceus) seeds, and 15.4 million ayu (Plecoglossus altivelis) seed fish annually. http://www.sinica.edu.tw/zool/zoolstud/43.2/165.pdf Key words: Brackish water ecosystem, Lake Nakaumi, Eutrophication, Mass seed production of fish, Economic value. Brackish waters, such as estuaries, salt es, and approximately 40% of such lake areas marshes, and mangrove swamps, are recognized have been reclaimed in the past few decades as ecosystems with the highest biological produc- (Hirai 1993). The perceived higher economic tion among aquatic ecosystems, owing to high value of newly reclaimed land compared to the nutrient inputs from the adjacent land and rivers original water body has been the major reason (i.e., Odum 1971). These areas are not only pro- that many people favor reclamation. In accor- ductive fish and shellfish fishery grounds, but also dance with increased public awareness of the eco- habitats for many migrating water bird species. logical importance of brackish water ecosystems From these aspects, brackish waters can be and environmental deterioration occurring therein, regarded as important ecosystems to be pre- more attention is now being paid to preserving served. However, in Japan, the major coastal these productive ecosystems. brackish lakes, such as Lake Kasumigaura and Two estuarine lagoons, Lake Shinji (with an Lake Hachirogata, have been desalinated and/or area of 79.2 km2 and an average depth of 4.5 m) reclaimed for agricultural and/or industrial purpos- and Lake Nakaumi (with an area of 86.8 km2 and *To whom correspondence and reprint requests should be addressed. 165 166 Zoological Studies 43(2): 165-172 (2004) an average depth of 5.4 m), and a short connect- are being operated, the landings were extremely ing river (with a length of 7.3 km) between the 2 reduced after the embankment was built. lakes, the Ohashi River, constitute the largest The first objective of this paper was to clarify brackish water ecosystem remaining in Japan at the biological production of mesozooplankton, the present (Fig. 1). Fresh water from the Hii River major secondary producers which graze on phyto- and other small streams and saltwater through the plankton and prey upon by planktivorous fish and Sakai Strait (with a length of 7.5 km, and a width of carnivorous invertebrates, in Honjo District. We 0.3 km) meet and mix in this area, providing oligo- conducted a monthly investigation to measure haline to polyhaline salinity gradients (Okuda mesozooplankton abundance, biomass, and pro- 1997). Due to both limited water exchange duction rates during 2 yr at 7 sampling stations in between Miho Bay and Lake Nakaumi and Honjo District, in addition to a station in adjacent increased load of nutrients and organic matter Lake Nakaumi (Fig. 1). If the potential exists to from urban sewage and agricultural fields, this mass-produce mesozooplankton in Honjo District, lagoonal system has become highly eutrophic. we would like to propose a plan to use this The formerly productive ecosystem has been con- enclosed brackish area as a site for intensive fish siderably deteriorated, particularly in Lake seed production. This would increase the eco- Nakaumi, as manifested by frequent phytoplankton nomic potential of this water body, and this has blooms and the concomitant formation of benthic never been done before. Hence, our second anoxia (Ohtake et al. 1982, Kondo et al. 1990). objective was to assess the feasibility of this plan. Lake Shinji, in contrast, continues to sustain a dense population of the commercial filter-feeding bivalve (Corbicula japonica), whose annual harvest MATERIALS AND METHODS is around 8000 t wet weight with the shell (Nakamura 1998). We visited 7 stations in Honjo District (stns. 1 In the 1970s, there was a plan to reclaim the to 7, with depths of 3-9 m) and a station (stn. 8, northwestern part of Lake Nakaumi (called the with a depth of 5 m) in Lake Nakaumi monthly dur- Honjo District, with an area of 16.2 km2, an aver- ing 2 yr from June 1997 to May 1999 using the age depth of 5.1 m, and a water volume of 8.25 x boat Gobius of Shimane University (Fig. 1). At 107 m3) for agricultural use, and consequently the each station, vertical profiles of temperature, salini- district was enclosed by construction of a bank in ty, and dissolved oxygen concentrations were 1981. The water exchange between Honjo District determined at every 1 m interval with a water and Lake Nakaumi per se is limited to that which meter (Horiba, U-10). Water samples were taken occurs through a narrow waterway along the west- with a Van Dorn water bottle from depths of 0.5, ern shore. Although fisheries, mainly by set nets, 4.0, and 6.0 m, and 50 ml of the water was filtered Sea of Japan Sakai Strait Miho Bay Honjo District St. 1 35°30'E Ohashi River Lake Nakaumi St. 3 Hii Lake Shinji St. 4 St. 2 River St. 5 05km St. 6 133°E St. 7 02km St. 8 Fig. 1. Map of the Lake Shinji, Ohashi River, and Lake Nakaumi brackish water system, with the location of sampling stations in Honjo District, an enclosed water body, and in Lake Nakaumi. Uye et al. -- Copepod Biomass and Production 167 onto glassfiber (Whatman GF/F) filters. (P, mg C m-3 d-1) was estimated based on its bio- Chlorophyll α concentrations of these seston sam- mass (B, mg C m-3) and an empirically-determined ples were determined fluorometrically (Turner potential (or maximum under non-food limitation) Designs, Model 10) after extraction in N,N- specific growth rate (g, d-1): P = B x g. The specif- demethylformamide. Mesozooplankton samples ic growth rates of copepods and appendicularians were collected with vertical hauls of a plankton net in relation to temperature have previously been (with a mouth diameter of 0.225 m, a length of 1 determined for species mainly from the Inland Sea m, and a mesh size of 100 µm) fitted with a flow of Japan (see table 1 of Uye and Shimazu 1997). meter (Rigosha Co.) from the bottom to the sur- For Acartia sinjiensis, the following equation to face. Plankton samples were immediately fixed express a relationship between growth rate and with 5% buffered formalin. temperature (T, C) was used: g = 0.022exp Zooplankton samples were split into 1/2 to (0.032T) (Uye and° Fujinaka, unpubl. data). We 1/128 portions, depending on the density of organ- assumed that Pseudodiaptomus inopinus has a isms, with a Motoda box splitter. From these sub- similar growth rate-temperature relationship to the samples, at least 200 specimens were identified congener, P. marinus. For the remaining taxa and counted under a stereoscopic binocular micro- (e.g., cladocerans, malacostracans, chaetognaths, , scope. Copepods (except for Harpacticoida), and benthos larvae), Ikeda-Motoda s physiological cladocerans, appendicularians, and chaetognaths method was applied (Ikeda and Motoda 1978; were identified to the species or genus level, while Omori and Ikeda 1984). other taxa, such as malacostracans and larvae of various benthic animals, were classified to higher taxonomic levels. Their appropriate body dimen- RESULTS sions (see Uye 1982) were measured using a video micrometer (Olympus, VM-10), and automa- Environmental variables tically converted, using a personal computer, to carbon weights using predetermined length-weight Since environmental variables were very simi- regressions (see Uye and Shimazu 1997 for lar among the 7 stations in Honjo District, we details). chose stn. 3 as a representative of this area. The production rate of each taxonomic group Seasonal variations at the surface (i.e., 0.5 m) and Table 1. Comparison of annual mean biomass and production rates of meso- zooplankton among various estuarine and coastal marine waters Location Biomass Production rate Reference (mg C m-3) (mg C m-3 d-1) Fukuyama Harbor, Japan 39.1 6.85 Uye and Liang 19981 Inland Sea of Japan, Japan 20.0 2.83 Uye and Shimazu 1997 Honjo District, Japan 47.4 - Uye et al.